The present invention relates to semiconductor power switch devices including thyristors, and, more specifically, such devices utilizing serial combinations of thyristors and diodes.
Conventional power switches generally employ symmetrically blocking thyristors with the zone sequence n.sup.+ pnp.sup.+. A symmetrically blocking thyristor is defined as a thyristor that can block voltages of at least approximately the same magnitude in the on-state as well as in the off-state direction. In the conducting state, the weakly doped central zones of the thyristor are swamped with charge carriers. If such a thyristor is blocked at that time, the carriers stored in the interior of the semiconductor body of the thyristor must be swept out. After the charge is swept out, the thyristor is able to withstand its blocking voltage. For this reason, the charge is called the reverse recovery charge Q.sub.rr and is a characteristic parameter of every thyristor. The value of the parameter depends essentially on the thickness of the central zone, i.e. on the maximum possible blocking voltage and is proportional approximately to the square of the thickness of the central zone. The quantity Q.sub.rr therefore increases at a greater rate than directly proportionally with increasing maximum blocking voltage.
The result of the stored charge is that immediately after current crossover the thyristor cannot block yet. A return current will therefore continue to flow through the thyristor until the charge carriers have been approximately cleared from the central zone. At approximately at the peak of the return current, the thyristor starts to receive the blocking voltage. When such thyristors are operated with inductive loads, a voltage which places the thyristor under stress in the blocking direction will build up at the inductance. The magnitude of this voltage is determined by the magnitude of the return current and hence by the magnitude of the reverse recovery charge as well as by the circuit parameters including the R-C effect of the wiring.
It would indeed be possible to reduce the storage charge by weaker doping of the outer zones of the thyristor. But then as a consequence,the conducting-state voltage of the thyristor increases and the power dissipation in the thyristor also rises.
It is an object of this invention to provide a semiconductor power switch device of the thyristor type in such a way that at a given blocking voltage the blocking delay charge and hence also the recovery time is reduced without increasing the conducting-state voltage U.sub.T.